Electronic devices, particularly integrated circuits and semiconductors, are usually coated or encapsulated with a polymeric silicone resin, such as a room temperature vulcanized (RTV) silicone elastomer, to protect the device from mechanical damage and from adverse environmental effects of temperature and humidity.
In the process of making the encapsulant, trace amounts of unwanted anions, particularly chloride ions, and HCl are generally introduced. For example, HCl is a by-product in the preparation of several of the materials used to make the encapsulant, in particular the base polymer, crosslinker, and catalyst. The acid is not adequately removed from these and is, therefore, subsequently introduced into the encapsulant. Chloride ions are introduced by the dissociation of NaCl, hydrolysis of HCl, and as a residue of solder flux. The presence of these contaminants leads to device malfunctioning and failure. HCl and chloride ions, in particular, cause corrosion of the metallic components of the electronic device, dendrite formation, and eventually short circuiting due to the formation of metallic dendritic bridges between the cathode and anode of the device.
Attempts to further reduce the concentration of contaminants present in encapsulating compositions by modifying the method of preparation have proven unsuccessful. Another approach has been to sequester the contaminants so that they are not later free to migrate under the conditions of high bias, temperature and moisture that typify the operating environment of many electronic devices. For example, Kaneda reported in Chem. Abstracts (March, 1976, p. 39929q, Japanese Pat. No. 76-11377), under the title "Semiconductor Sealing Resin Composition Containing Crown Ethers or Cryptate Ethers as the Alkali Metal Ion Getters," that the addition of crown ethers or cryptate ethers resulted in better operational stability of resin-molded semiconductor devices by forming stable complexes with alkali metal ion contaminants. However, the formulation Kaneda discloses is limited to the sequestering of cations.
Accordingly, it is an object of this invention to provide a method of preventing migration of anions on an electronic device by immobilizing the anions.